1. Background--Field of Invention
This invention provides methods to measure and control the release of electric charge in the turbine and exhaust hood of a steam power generating unit, whereby harmful electrical discharges that cause corrosion, induce turbulence and decrease power output may be decreased.
2. Background--Discussion of Prior Art
HUEBNER, U.S. Pat. No. 3,859,005 teaches that the size of water droplets in the wet steam flowing through the last stages of a turbine may be reduced and erosion within the turbine thereby decreased by applying an electric potential to stationary surfaces within the turbine and, optionally, also applying an electrical potential to the rotating turbine blades. HUEBNER makes no mention of naturally occurring electrostatic charge in the wet steam, and does not talk about the effects of electrostatic charge in the exhaust hood and condenser of the generating unit.
3. Background--Case Study of Harmful Electrostatic Effects
While we believe the explanations given here to be true, we do not wish to be bound by them. Terms and symbols are defined in the section "Background--Definition of Terms", below.
In U.S. patent application Ser. No. 08/589,420 TARELIN et al. reported measurements of electric charge density in the wet steam exiting a 50 MW turbine in an electric power plant in Ukraine (referred to as "Unit A" hereinafter) indicating a positive electric charge density in the wet steam .rho..sub.fg =+1mC m.sup.-3 =+1nC cm.sup.-3.
Similar measurements of electric charge density have been carried out in a modern 800 MW electric power generating unit in the United States (referred to as "Unit B" hereinafter). Both generating units were operating on "All Volatile Treatment", wherein ammonia was added to the feedwater. The pH values and chemical compositions of feedwater and moisture at Units A and B are further discussed in Example 2, below. The method of charge measurement employed is described in Example 1. The design of Unit B is described in Example 5.
Prior to November 1996, ammonia concentration of 800-1400 ppb was maintained in the feedwater of Unit B with feedwater pH in the interval 9.2-9.6, most commonly about 9.2. Since November 1996, the concentration of ammonia in the feedwater has been maintained at 70-90 ppb with feedwater pH in the interval 8.0-8.5. Oxygen has also been added to maintain 60-150 ppb oxygen in the feedwater.
During an outage in March 1997, large corroded areas were first observed on the condenser walls of Unit B in areas where the steam flows down along the walls, and most of the tubular braces inside the condenser of Unit B were severely corroded on their upper surfaces. The corroded surfaces were clean, shiny metal with an etched appearance and feel to it. The outer flow guides of the low pressure turbine were mostly covered with a thin layer of red iron oxide. Most surface analyses of the turbine buckets, nozzles, casings, and other turbine components exposed to steam flow indicated a surface layer consisting mostly of silicon dioxide with iron and other elements present in smaller amounts. The silica contamination is derived from silica carry-over in the steam and is commonly observed in many generating units.
The electric charge carried by water droplets in the turbine exhaust is produced by flow electrification (Loeb, 1958). If ammonia or another volatile base (for example, morpholine) is present in the steam, some of the base will dissolve in the condensate film that forms on the turbine blades and other surfaces within the turbine. If pH.sub.M &gt;IEP (isoelectric point) of the oxide film upon the metal surface, the metal surface will release protons to the alkaline condensate. The surface of the metal will have a slight negative charge, which is matched by the positive charge of the diffuse double layer consisting of ammonium ions. Due to the dilute nature of the condensate, the diffuse positive charge extends some distance into the liquid phase (.kappa..sup.-1 in Table 1 represents this distance; charge density in the diffuse double layer decreases by a factor of e.sup.-1 =0.367879 . . . over a distance of .rho..sup.-1). When liquid drops are torn from the turbine blades by the rapidly flowing steam, the outer portion of the diffuse double layer is torn away with the drop. The drop thereby acquires a positive charge. High velocity flow of wet steam in contact with other metal surfaces (for example, the nozzles, turbine casing, flow guides, etc.) can also produce electrically charged water droplets. The extreme purity of the liquid water in the wet steam allows large charge separation to occur, because the ability of charge to return through the water to the surface of the metal is limited by the very small conductivity of the condensate.
A negative charge can also be released to the wet steam. The metal oxide-covered metal surfaces usually are amphoteric, and their acid-base properties are primarily determined by the value of IEP, which is different for different materials. Values of IEP for several metal oxides and minerals are summarized in Table 2. If pH.sub.M &lt;IEP, the metal surface will have a positive net charge, the charge of the diffuse double layer will be negative, and the wet steam will acquire a negative charge. If pH.sub.M =IEP, the surface has no net charge, there is no diffuse double layer, and water droplets separated from the surface of the metal will have no electric charge.
A simple quantitative model of the charge release process is presented in Example 3. This model predicts charge densities of the magnitude actually observed at Unit A and Unit B. It predicts that the positive charge of the wet steam will increase with increasing pH.sub.M and pH.sub.FW, while the release of negative charge will decrease with increasing pH.sub.M and pH.sub.FW.
In Unit B, it was found that positive charge was released from the turbine; as predicted the amount of positive charge released increased with increasing pH.sub.M and pH.sub.FW (Example 4). Because silica has a very low IEP (Table 2), the presence of silica contamination on metal surfaces inside the turbine readily explains the large amount of positive charge coming out of the turbine at the higher pH values. Negative charge was released from the flow guides and other carbon steel members exposed to high velocity steam flow; as predicted, the amount of negative charge released decreased with increasing pH.sub.M and pH.sub.FW. At pH.sub.FW near to 9.3 the positive charge released from the turbine approximately equaled the negative charge released from the flow guides. Charge density values measured down in the condenser were relatively small, and both positive and negative values were recorded. At pH.sub.FW &lt;9, negative charge predominated, and substantial negative charge densities were recorded in the upper part of the condenser. These measurements are described in Example 6.
The large negative charge that predominates in the condenser traverses at pH.sub.FW &lt;9 is probably released by the flow guides and similar members, which are exposed to nearly the same flow velocities as the turbine blades, and have sharp downstream edges, which are conducive to good charge release. The flow guides are made of carbon steel and are covered with a red iron oxide; if the iron oxide present is hydrous Fe.sub.2 O.sub.3 (that is, common rust) with IEP=8.6 (Table 1) the release of negative charge is to be expected, as pH.sub.M &lt;IEP throughout the range of ammonia concentrations of interest (Table 1). Because the presence of oxygen in the steam would favor the formation of rust on carbon steel surfaces, oxygen may also be a factor in releasing the large amount of negative charge observed.
The maximum values of voltage measured on the traverse probes indicates a break-down electric field strength E.sub.bd =1.3 kV/cm=130 kV/m within the condenser (Example 7). The electric field strength calculated for four of the traverses exceed this value by a large margin, much larger than the likely error in all of the approximations that went into the calculation. It is obvious that electrical discharges are taking place at least in the region that extends from the turbine to the Perpendicular Traverse. At the very large electric field strengths indicated, arc discharges are to be expected, probably resembling little lightening bolts hitting the metal surfaces exposed to the flow. The presence of characteristic corrosion on the upper surfaces of virtually all support braces, even those just above the tube bundle and those down between the tube bundles suggests some degree of electrical discharge activity at all metal surfaces exposed to rapid steam flow.
The calculated charge densities at Unit B summarized in Table 3 are smaller than the +1mC m.sup.-3 measured at Unit A by a factor of ten or more. The steam leaving the turbine of Unit B is immediately deflected by the several flow guides and baffles, and then flows down the end walls and past the numerous support braces before it gets to the tube bundles. The space charges is rapidly dissipated by discharges to these grounded metal surfaces, as evidenced by the progressive decrease of total estimated current as the tube bundle is approached (Table 3).
At pH.sub.FW .gtoreq.9, immediately downstream of the turbine there exists a plume of positive charge released from the turbine surrounded by a zone of negative charge released from the outer flow guide. This zone contains intense electrical discharge activity and turbulence, and the opposite charges dissipate rapidly, consistent with the relatively small charge densities (both positive and negative) observed along the diagonal and perpendicular traverses further down in the condenser.
Release of the positive space charge from the turbine blades produces a moderate anodic polarization along the blade edges where the charged water droplets leave the edge of the blade. This anodic polarization may favor SCC on the trailing edges of the L-stage turbine blades if the current density is large enough. Release of negative space charge produces a moderate cathodic polarization where the charged droplets leave the edges of the flow guides. This moderate cathodic polarization may produce some degree of hydriding at the edges of the flow guides.
Discharges from zones of positive space charge to ground will produce small areas of large cathodic polarization and high current density which persist for a very short time in any one spot. These cathodic "strikes" are likely to be important on the turbine blades and portions of the flow guides at high pH, emanating from the positive charge in the wet steam coming out of the turbine. These discharges may cause hydriding or another form of metal damage associated with cathodic polarization concentrated near to the trailing edges. The damage to stainless steel is not likely to be large in this situation, but titanium alloy blades could be damaged severely.
Discharges of negative space charge to grounded surfaces will produce spots of large anodic cathodic polarization and high current density on the grounded metal surface. This mechanism is consistent with the severe metal wastage observed on the carbon steel braces and condenser walls at Unit B since pH.sub.FW was decreased in late 1996. Brass would likewise be severely damaged if exposed to these discharges; for example, brass condenser tubes. Cupronickel condenser tubes might also be damaged. On the other hand, stainless steel and titanium would be damaged little if at all.
The electric field strength is greatest adjacent to a grounded metal surface, and the dissipation of charge will be most rapid, as evidenced by the drop-off in charge density at small values of x in the traverses (FIGS. 6 and 7). With this charge distribution, electrostatic forces induce convection; parcels of fluid with high charge density located at some distance from the metal surface will move toward the surface, displacing parcels of fluid near to the surface which have low charge density. The amount of electrostatic energy dissipated as kinetic energy of turbulence may exceed the amount of energy dissipated through electrical discharge because electrical discharge preferentially removes charge from zones closer to the surface where the electric potential is relatively low, while turbulence removes charge from throughout the profile. This means that at pH.sub.FW &lt;8 hundreds of kW will be dissipated by electrical discharge, and hundreds of kW as kinetic energy of turbulence. The turbulent motion created will predominantly be perpendicular to the solid surface, thereby favoring the rapid transport of momentum from the bulk fluid to the solid surface. Thus, frictional drag between flowing steam and the solid surface is increased, increasing pressure losses between the turbine and the tube bundles.
The large average velocity of steam leaving the turbine represents substantial kinetic energy (Table 3); turbulence in the steam contains additional kinetic energy. The kinetic energy of the turbulent flow feeds on the average flow, and may far exceed the electrostatic energy that initiates turbulence. The average flow velocity of the steam just above the tube bundle is largely recovered as static pressure as the flow impinges the tube bundle and condenses. The kinetic energy of turbulent flow perpendicular to the average flow will be recovered much less efficiently, if at all. The useless dissipation of most of the kinetic energy associated with turbulence comprises a large efficiency loss related to the electrostatic discharge.